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标题：New constraints on the P– T path of HT/UHT metapelites from the Highland Complex of Sri Lanka
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作者：P.L. Dharmapriya ; Sanjeewa P.K. Malaviarachchi ; L.M. Kriegsman 等
期刊名称：Geoscience Frontiers
印刷版ISSN：1674-9871
出版年度：2017
卷号：8
期号：6
页码：1405-1430
DOI：10.1016/j.gsf.2016.12.005
语种：English
出版社：Elsevier
摘要：AbstractWe report here rare evidence for the early progradeP–Tevolution of garnet–sillimanite–graphite gneiss (khondalite) from the central Highland Complex, Sri Lanka. Four types of garnet porphyroblasts (Grt1, Grt2, Grt3and Grt4) are observed in the rock with specific types of inclusion features. Only Grt3shows evidence for non-coaxial strain. Combining the information shows a sequence of main inclusion phases, from old to young: oriented quartz inclusions at core, staurolite and prismatic sillimanite at mantle, kyanite and kyanite pseudomorph, and biotite at rim in Grt1; fibrolitic sillimanite pseudomorphing kyanite ± corundum, kyanite, and spinel + sillimanite after garnet + corundum in Grt2; biotite, sillimanite, quartz ± spinel in Grt3; and ilmenite, rulite, quartz and sillimanite in Grt4.The pre-melting, original rock composition was calculated through stepwise re-integration of melt into the residual, XRF based composition, allowing the early prograde metamorphic evolution to be deduced from petrographical observations and pseudosections. The earliest recognizable stage occurred in the sillimanite field at around 575 °C at 4.5 kbar. Subsequent collision associated with Gondwana amalgamation led to crustal thickening along aP–Ttrajectory with an average dP/dTof ∼30 bar/°C in the kyanite field, up to ∼660 °C at 6.5 kbar, before crossing the wet-solidus at around 675 °C at 7.5 kbar. The highest pressure occurred atP > 10 kbar andTaround 780 °C before prograde decompression associated with further heating. At 825 °C and 10.5 kbar, the rock re-entered into the sillimanite field. The temperature peaked at 900 °C at ca. 9–9.5 kbar. Subsequent near-isobaric cooling led to the growth of Grt4and rutile atT∼880 °C. Local pyrophyllite rims around sillimanite suggest a late stage of rehydration atT < 400 °C, which probably occurred after uplift to upper crustal levels. U–Pb dating of zircons by LA-ICPMS of the khondalite yielded two concordant206Pb/238U age groups with mean values of 542 ± 2 Ma (MSWD = 0.24, Th/U = 0.01–0.03) and 514 ± 3 Ma (MSWD = 0.50, Th/U = 0.01–0.05) interpreted as peak metamorphism of the khondalite and subsequent melt crystallization during cooling.Graphical abstractDisplay OmittedHighlights•Preservation of textural evidence for early prograde to late retrogradeP-Tpath has been presented.•First attempt ofP-Tcalculation using thermodynamic modeling of early prograde P-T trajectory followed by the Highland Complex metasediments.•Peak metamorphicP-T-tconditions are around ∼9 kbar, 900 °C and 540 Ma respectively.•Implications ofP-T-tevolution of metasediments in the Highland Complex of Sri Lanka.
关键词：KeywordsenPrograde evolutionMelt re-integrationHighland ComplexSri LankaUHT granulites